Spot welding is a metal fabrication technique in which contacting metal surfaces are joined by the heat obtained from resistance to electric current. Work-pieces are held together under pressure exerted by electrodes. Typically the individual sheets are between 0.5 to 4 mm in thickness. The process uses two shaped typically copper alloy based electrodes to concentrate welding current into a small “spot” and to simultaneously clamp the sheets together. Forcing a large current through the spot will melt the metal and form the weld. After the material is heated the current is turned off allowing the melted material to cool while still under the pressure of the electrodes forming a solid “Spot” weld in that location. Testing the strength of the weld may include a destructive test or a non-destructive test. Destructive testing includes determining the amount of force necessary to separate the joined parts. Destructive tests, however, are contraindicated where destroying the workpiece is not ideal or cost-effective. Moreover, destructive tests do not allow for remedial actions to be taken during the welding process. Non-destructive testing includes identifying characteristics of the joint itself to determine its strength. Robust non-destructive tests are only available for particular substrate materials (i.e., steel), however, and not for other types of metal substrates such as aluminum.